The present,invention relates to mass flow controllers.
Mass flow controllers are known in the art for controlling the specific amount of flow of a fluid, necessary for a particular process, e.g., in semiconductor manufacturing processes, such as chemical vapor deposition or the like. Mass flow controllers are known to be capable of sensing the flow occurring through the controller and modifying or controlling that flow as necessary to achieve the required control of the mass of the fluid delivered to the particular process.
Sensing the flow is a function of the type of fluid utilized and the physical effect used to sense the amount of flow. One typical type of physical effect to sense mass flow is to measure the temperature differential between the upstream and downstream heater/sensor coils exposed to the fluid flow. Other systems may use absolute and/or differential pressure changes, light absorption, or the momentum change (e.g., paddle wheel) to measure the flow.
Modifying or controlling the flow is typically made in response to the sensed flow as it relates to the desired flow by modifying a cross-sectional opening area available to the fluid for flowing. The smaller the area available for flow, the smaller the mass flow, and vice-versa. In the past, this has been accomplished with a typical plunger/diaphragm/orifice system. An orifice provides the variable cross sectional opening area for flow, where the flow control is dictated by the positioning and motion of a plunger/diaphragm or needle stem in the orifice in response to a flow control signal. The flow control signal is generated in response to the measurement of the flow sensor.
A servo control section generates a control signal that drives the positioning of the plunger/diaphragm or needle stem, typically through the use of a solenoid type of driver. The solenoid driver has a ferromagnetic core surrounded by a coil. The plunger/diaphragm, typically made of ferromagnetic material, is held close to the orifice by a spring. The energizing of the coil generates a magnetic field that pulls the plunger/diaphragm away from the orifice while the spring pulls it toward the orifice. The distance between the orifice and the plunger/diaphragm is dependent upon the relative strengths of the magnetic field and the spring. The proportional control valve by its nature is not an open and shut valve. The closer the needle stem or plunger/diaphragm is to the orifice, the more restricted the flow becomes, until the flow is shut off, and the more it is withdrawn the more the flow increases, until it no longer affects the amount of flow.
For precision control, complex and expensive controller circuitry is needed to control the positioning and movement of the needle stem or plunger/diaphragm as the flow is regulated. The valve parts themselves must be manufactured with high precision, and are therefore expensive. In addition, prior art proportional controlled solenoid valve mass flow controllers require the needle stem or plunger/diaphragm to be mounted at right angles to the fluid flow direction. Consequently, the orifice is also mounted at right angles to the fluid flow path, and the fluid has to change direction to go through the orifice, which generates turbulence in the fluid.
Often the mass flow controller, particularly when used in high precision semiconductor manufacturing processes and the like, is part of a tool that has limited space available for the flow controllers, particularly if there are multiple mass flow controllers that are positioned in the immediate area of the actual discharge of the fluid into the tool""s process chamber.
There is a need in the art, therefore, for a mass flow controller that is simpler, less expensive, smaller, and easier to manufacture and control.
The present invention, according to one embodiment, utilizes a closed loop magnetic flux path passing through the body of the controller in the direction of flow from its input to its output to magnetically operate a flexible plunger button valve assembly that is normally spring biased into the shut position. A current generated from a servo control section of a mass flow controller generates magnetic flux to pull the plunger valve assembly away from an orifice and allow more fluid to flow through. By controlling the amount of flux generated, and thereby the positioning of the button valve assembly relative to the orifice, the flow through the orifice can be controlled. Consequently, a large separate proportional control valve section is no longer necessary, which results in a more compact, less expensive and more reliable mass flow controller that is less costly to manufacture and has fewer components than the conventional mass flow controllers discussed above.